Superluminescent diodes (SLDs) are optical devices that provide amplified spontaneous emission outputs confined to one spatial mode. The spatial distribution of the output light is similar to a laser while the spectral distribution is similar to an LED. SLDs are often specified for applications requiring high beam quality, but where the narrow linewidth of the laser is undesirable or detrimental. An SLD typically has a structure similar to that of a laser, with lasing being prevented by antireflection coatings formed on the end faces. One such device is described in U.S. Pat. No. 4,821,277, which is incorporated herein by reference and which is characterized by a tilted waveguide structure. The axis of symmetry of the waveguide is formed at an angle relative to the direction perpendicular to at least one of the end faces and the tangent of the angle is greater than or equal to the width of the effective optical beam path divided by the length of the body between the end faces.
The spectral width of the SLD is determined by the emission of spectrum, and is typically 20 to 40 nanometers for SLDs emitting light between 0.7 to 1.5 micrometers in wavelength. In a typical SLD, such as described in the U.S. Pat. No. 4,821,277 and U.S. Pat. No. 6,417,529, which is also incorporated herein by reference, the device emits light at wavelengths equivalent to or somewhat shorter than the quantum well wavelength. This wavelength corresponds to the difference in the quantum well electron and hole energy levels. The density of states in the quantum well and the thermal distribution of electrons and holes populating the states are the primary determinants of the emission spectral width. These factors are difficult to alter without, for example, using quantum dots as the active medium.
Methods and devices for broadening or extending the spectral width of an SLD are described in U.S. Pat. No. 6,184,542, incorporated herein by reference. The devices described have emission layers disposed side-by-side. Light emitting from an emission layer has a longer wavelength than light emitted from an adjacent layer in the direction of emission. The light of the different wavelengths results in a broadened output spectra at a point beyond the adjacent layer. A narrower spectral width is sometimes needed; however, existing methods and devices fail to provide a satisfactory and repeatable way to narrow the spectral output of a SLD.